Few-cycle Laser Pulses Research Articles

Unconventional two-step spin relaxation dynamics of [Re(CO)3(im)(phen)]+ in aqueous solution.

Changes of molecular spin are ubiquitous in chemistry and biology. Among spin flip processes, one of the fastest is intersystem crossing (ISC) in transition metal complexes. Here, we investigate the spin relaxation dynamics and emission spectrum of [Re(CO)3(im)(phen)]+ (im = imidazole, phen = phenanthroline) using extensive full-dimensional excited-state dynamics simulations in explicit aqueous solution. Contrary to what has been observed in other transition metal complexes, the transition from the singlet to triplet states occurs via a two-step process, with clearly separable electronic and nuclear-driven components with two different time scales. The initially excited electronic wave function is a "molecular spin-orbit wave packet" that evolves almost instantaneously, with an 8 fs time constant, into an approximate 25 : 75 singlet-to-triplet equilibrium. Surprisingly, this ISC process is an order of magnitude faster than it was previously documented for this and other rhenium(i) carbonyl diimine complexes from emission spectra. Simulations including explicit laser field interactions evidence that few-cycle UV laser pulses are required to follow the creation and evolution of such molecular spin-orbit wave packets. The analysis of the dynamics also reveals a retarded ISC component, with a time constant of 420 fs, which can be explained invoking intramolecular vibrational energy redistribution. The emission spectrum is shown to be characterized by ISC convoluted with internal conversion and vibrational relaxation. These results provide fundamental understanding of ultrafast intersystem crossing in transition metal complexes.

Asymmetry flip in photoelectron angular distribution of rare gas atoms in intense circularly-polarized few-cycle laser fields

An analytical formula representing the photoelectron kinetic energy at which the ejection direction of photoelectrons generated by an intense circularly-polarized few-cycle laser pulse flips was derived and was used for determining the laser pulse duration.

Generation of few-cycle laser pulses by coherent synthesis based on a femtosecond Yb-doped fiber laser amplification system

We demonstrate a coherent synthesis system based on femtosecond Yb-doped fiber laser technology. The output pulse of the amplification system is divided into two replicas and seeded into photonic crystal fibers of two parallel branches for nonlinear pulse compression. Because of the different nonlinear dynamics in the photonic crystal fibers, the compressed pulses show different spectra, which can be spliced to form a broad coherent spectrum. The integrated timing jitter between the pulses of two branches is less than one tenth of an optical cycle. By coherently synthesizing pulses from these two branches, 8 fs few-cycle pulses are produced.

Central frequency of few-cycle laser pulses in strong-field processes

We analyze the role of the difference between the central frequencies of the spectral distributions of the vector potential and the electric field of a short laser pulse. The frequency shift arises when the electric field is determined as the derivative of the vector potential to ensure that both quantities vanish at the beginning and end of the pulse. We derive an analytical estimate of the frequency shift and show how it affects various light induced processes, such as excitation, ionization and high harmonic generation. Since observables depend on the frequency spectrum of the electric field, the shift should be taken into account when setting the central frequency of the vector potential to avoid potential misinterpretation of numerical results for processes induced by few-cycle pulses.

Asymmetry flip in angle-resolved photoelectron spectra of rare gas atoms by intense circularly polarized few-cycle pulses

We examine the phenomenon of the flip in the asymmetry in the emission direction of photoelectrons from rare gas atoms upon the ionization by a circularly polarized few-cycle laser field, and derive an analytical formula representing the photoelectron kinetic energy at which the asymmetry flip occurs based on the semiclassical tunnel ionization theory. We show that the energy at which the asymmetry flip occurs varies sensitively to the duration of few-cycle laser pulses and propose that the pulse duration can be determined experimentally using this analytical formula for the asymmetry flip energy.

Deformation of Atomic p_{±} Orbitals in Strong Elliptically Polarized Laser Fields: Ionization Time Drifts and Spatial Photoelectron Separation.

We theoretically investigate the deformation of atomic p_{±} orbitals driven by strong elliptically polarized (EP) laser fields and the role it plays in tunnel ionization. Our study reveals that different Stark effects induced by orthogonal components of the EP field give rise to subcycle rearrangement of the bound electron density, rendering the initial p_{+} and p_{-} orbitals deformed and polarized along distinctively tilted angles with respect to the polarization ellipse of the EP field. As a consequence, the instantaneous tunneling rates change such that for few-cycle EP laser pulses the bound electron initially counterrotating (corotating) with the electric field is most likely released before (after) the peak of the electric field. We demonstrate that with a sequential-pulse setup one can exploit this effect to spatially separate the photoelectrons detached from p_{+} and p_{-} orbitals, paving the way towards robust control of spin-resolved photoemission in laser-matter interactions.

Coherent Electron Trajectory Control in Graphene.

We investigate coherent electron dynamics in graphene, interacting with the electric field waveform of two orthogonally polarized, few-cycle laser pulses. Recently, we demonstrated that linearly polarized driving pulses lead to sub-optical-cycle Landau-Zener quantum path interference by virtue of the combination of intraband motion and interband transition [Higuchi etal., Nature 550, 224 (2017)NATUAS0028-083610.1038/nature23900]. Here we introduce a pulsed control laser beam, orthogonally polarized to the driving pulses, and observe the ensuing electron dynamics. The relative delay between the two pulses is a tuning parameter to control the electron trajectory, now in a complex fashion exploring the full two-dimensional reciprocal space in graphene. Depending on the relative phase, the electron trajectory in the reciprocal space can, e.g., be deformed to suppress the quantum path interference resulting from the driving laser pulse. Intriguingly, this strong-field-based complex matter wave manipulation in a two-dimensional conductor is driven by a high repetition rate laser oscillator, rendering unnecessary complex and expensive amplified laser systems.

High-order harmonic generation of the heteronuclear hydrogen molecular ion in strong infrared laser fields

The high harmonic generation of heteronuclear hydrogen molecular ions in few-cycle infrared strong laser fields is studied by numerically simulating the time-dependent Schr\odinger equation. By reversing the carrier envelope phase of the few-cycle laser pulse, the electron dynamics is reversed as well. However, due to the different nuclear masses, two nuclei move with different repulsive speeds, which makes the rescattering processes not identical when the carrier envelope phase shifts by $\ensuremath{\pi}$. The different rescattering processes lead to distinct harmonic spectra in the cutoff region. The mass-dependent harmonic spectrum offers a view angle to diagnose the isotope dynamics in molecules with unprecedent high time and spatial resolutions.

Electron acceleration driven by sub-cycle and single-cycle focused optical pulse with radially polarized electromagnetic field.

The space-time properties of the expressions of sub-cycle and single-cycle focused optical pulses with radially polarized electromagnetic field based on the Sink-Source model are studied. The self-induced blue shift of the center frequency of spectrum in the center of the pulse field is found to have an important impact on the electrons acceleration. When the electrons approach to the center of pulse, the electrons will obtain a large kinetic energy gain in a short time. The effect of radiation-reaction force can't be ignored if the net kinetic energy gain of electrons is more than GeVs. The electrons will deviate from the original acceleration channel and the gain of kinetic energy that electrons may gain will be greatly reduced if the radiation-reaction effect is considered. In contrast to the few-cycle laser pulse accelerating electrons, the gain of kinetic energy obtained by electrons is a few times higher and the corresponding peak optical power is one order of magnitude lower in the case of the sub-cycle laser pulses accelerating electrons. The maximal kinetic energy gain of electrons is robust against the variation of the incident angles.

Experimental observation of anharmonic effects in coherent phonon dynamics in diamond

We present a study of coherent optical phonon dynamics in monocrystalline and polycrystalline diamond using transient transmission spectroscopy with few-cycle laser pulses. Besides the oscillations at the fundamental phonon frequency of 40 THz we observe a signal at second harmonic frequency and four weak frequency sidebands (25.0 THz, 32.5 THz, 47.5 THz and 55.0 THz). Potential origin of these sidebands is discussed along with the dependence of the observed signal on experimental conditions (pump power, sample thickness, etc.). Phonon dephasing time in polycrystalline diamond T2,poly = 6.0 ± 0.4 ps is found to be shorter than in monocrystalline diamond T2,mono = 7.0 ± 0.4 ps.

Proposal for CEP measurement based on terahertz air photonics

Single-shot carrier envelope phase (CEP) measurement is a challenge in the research field of ultrafast optics. We theoretically investigate how an intense terahertz pulse modulates second harmonic emission (SH) from a gas plasma induced by a few-cycle laser pulse (FCL). Results show that the modulation quantity of SH intensity has a cosinoidal dependence on the CEP of FCL pulses, based on which we propose a low energy, all-optical method for single-shot CEP measurements via using a known intense terahertz pulse. Moreover, we propose an experimental realization.

Separating intracycle interferences in photoelectron momentum distributions by a polarization-gated pulse

We theoretically demonstrate the feasibility of separation of various of intracycle interferences in photoelectron momentum distributions (PMDs) with a polarization-gated (PG) pulse constructed by a left-circularly polarized and a time-delayed right-circularly polarized few-cycle laser pulse. By numerically solving the time-dependent Schrodinger equation, our simulations show that the intercycle interference pattern is suppressed and a clear interference pattern with rightward curved stripes emerges in PMDs with appropriate laser parameters. This interference pattern can be well reproduced by a quantum-trajectory Monte Carlo (QTMC) simulation and, moreover, with the help of QTMC analysis, up to three types of intracycle interferences can be identified. It is further revealed that the nonvanishing transverse electric-field component in the PG pulse plays an important role in separating various of intracycle interferences in PMDs by properly distributing the interference stripes of electron wave packets, emitted within different time windows, into different momentum regions and suppressing the forward-rescattering photoelectron holography.

Femtosecond pulse compression using a neural-network algorithm.

A key requirement for femtosecond spectroscopy measurements is to compress the laser pulse to its transform-limited duration. In particular, for few-cycle laser pulses, the compression process is time-consuming using conventional algorithms that converge statistically. Here we show that machine learning can accelerate the process of pulse compression: we have developed an adaptive neural-network algorithm to control a deformable-mirror-based pulse shaper that converges 100× faster than a standard evolutionary algorithm.

Energetic electron bunch generation by laser interaction with xenon clusters

We study the interaction of intense, sub-cycle, and few-cycle laser pulses with xenon clusters for the generation of mono-energetic electron bunches. For this purpose, we used three dimensional, relativistic, molecular dynamics simulations. In this work, we used two mutually perpendicularly polarized (MPP) pulses separated by a finite temporal phase delay. The first pulse is responsible for the generation of electrons by field ionization of atomic clusters. However, the second pulse tends to accelerate the electrons (created by the first pulse) as a bunch. The effect of phase delay, pulse duration, and peak laser intensity on the generation of energetic electron bunches is studied. Under optimum conditions, the electrons are found to be accelerated to energies as high as 2.5 MeV. The feasibility of further acceleration of these electron bunches utilizing laser wakefield acceleration is also explored in this work by treating the accelerated electron bunch by MPP pulses as an initial condition to the nonlinear one-dimensional laser wakefield equations. The rough estimate of the final accelerated electron energies after laser wakefield acceleration has also been made.

Coherent extreme-ultraviolet emission generated through frustrated tunnelling ionization

Coherent extreme-ultraviolet emission can be obtained through high-harmonic generation and multiphoton excitation from atoms exposed to a strong laser field. We report the generation of a new kind of coherent extreme-ultraviolet emission from He atoms excited by intense few-cycle laser pulses. An atom can be excited after tunnelling in a strong laser field, in the process known as frustrated tunnelling ionization (FTI). We find that excitation through FTI leads to coherent extreme-ultraviolet emission, and its intensity strongly depends on the ellipticity and carrier-envelope phase of the laser pulses. Additionally, the propagation direction of the emission can be coherently controlled by employing the attosecond lighthouse technique. This coherent control of tunnelling and recombination dynamics, which has provided the fundamental basis of attosecond physics, promises the utilization of FTI emission as a coherent light source and offers new opportunities in ultrafast spectroscopy. Coherent extreme-ultraviolet emission through frustrated tunnelling ionization is observed from He atoms excited by intense few-cycle infrared laser pulses. Its intensity depends on the ellipticity and the carrier-envelope phase of the infrared laser.

Generation of low power and ultrashort laser pulses at 800 nm through soliton compression in chloroform-infiltrated cascaded photonic crystal fibers

A unique modest approach to obtain low energy, few-cycle laser pulses at 800 nm through chloroform filled cascaded photonic crystal fiber (PCF) is theoretically presented. Temperature is considered as the control parameter in designing the cascaded PCF to compress femtosecond pulses using higher order soliton compression scheme. The temperature-dependent parameters of both fiber and the infiltrated chloroform liquid are utilized to solve the modified nonlinear Schrödinger equation that facilitates the engineering of ultrashort pulses. Through this method, cascaded PCF could be designed using a single fiber for the generation of short pulses with minimal pedestal energy deprived of any other supplementary modules. We numerically prove that the compression of pulses from an input pulse of 140 fs up to 6.98 fs is possible by using temperature-controlled PCF. The designed fiber compressor is estimated to provide the possible compression factor of 20.05, quality factor of 0.49, output energy of 4.81 pJ, with 50.81% of pedestal energy, through 2.4 cm of entire length.

Metasurface-based multi-harmonic free-electron light source

Metasurfaces are subwavelength spatial variations in geometry and material where the structures are of negligible thickness compared to the wavelength of light and are optimized for far-field applications, such as controlling the wavefronts of electromagnetic waves. Here, we investigate the potential of the metasurface near-field profile, generated by an incident few-cycle pulse laser, to facilitate the generation of high-frequency light from free electrons. In particular, the metasurface near-field contains higher-order spatial harmonics that can be leveraged to generate multiple higher-harmonic X-ray frequency peaks. We show that the X-ray spectral profile can be arbitrarily shaped by controlling the metasurface geometry, the electron energy, and the incidence angle of the laser input. Using ab initio simulations, we predict bright and monoenergetic X-rays, achieving energies of 30 keV (with harmonics spaced by 3 keV) from 5-MeV electrons using 3.4-eV plasmon polaritons on a metasurface with a period of 85 nm. As an example, we present the design of a four-color X-ray source, a potential candidate for tabletop multicolor hard X-ray spectroscopy. Our developments could help pave the way for compact multi-harmonic sources of high-energy photons, which have potential applications in industry, medicine, and the fundamental sciences.

Ultrafast modification of band structure of wide-band-gap solids by ultrashort pulses of laser-driven electron oscillations

The electric field of high-intensity ultrashort laser pulses substantially perturbs an electron subsystem of a crystal and affects its band structure. Laser-driven oscillations of electrons and holes are frequently referred to as a major mechanism of the perturbation in dielectrics and semiconductors. New physical effects arise when the band structure is modified by an ultrashort pulse of the oscillations driven by a few-cycle laser pulse. Assuming the laser-pulse envelope varies slowly compared to the carrier frequency, we derive analytical relations for the laser-modified band structure by utilizing the Keldysh cycle-averaged nonperturbative approach under the approximation of constant effective mass. Formation of indirect-gap transient bands, suppression of the nonlinear absorption on the leading edge of a laser pulse, and cycle-averaged photocurrent generation driven by the pulse envelope are predicted. Analytical scaling with six laser and material parameters is obtained. The reported results establish the limits of validity of the Keldysh photoionization model and advance understanding of the fundamental effects involved in high-intensity ultrafast laser-solid interactions.

All-optical characterization of the two-dimensional waveform and the Gouy phase of an infrared pulse based on plasma fluorescence of gas.

The characterization of the temporal waveform of few-cycle laser pulses is an indispensable part in strong-field physics and attosecond science. Recently, a simple waveform-characterization technique called TIPTOE (tunneling ionization with a perturbation for the time-domain observation of an electric field) has been demonstrated for measuring linearly polarized few-cycle pulses. We theoretically and experimentally show that TIPTOE can be extended to resolve more characteristics of an optical waveform: the two-dimensional polarization and the Gouy phase. Based on the plasma fluorescence of a gaseous medium, we achieve all-optical and spatially resolved measurements of the waveform of an infrared pulse. This detection method enables the remote characterization of a waveform without the need to place an apparatus near the focal point of the laser beam. The proposed approach represents a simple and powerful method for conducting waveform diagnostics on few-cycle laser sources.

Continuous every-single-shot carrier-envelope phase measurement and control at 100 kHz.

With the emergence of high-repetition-rate few-cycle laser pulse amplifiers aimed at investigating ultrafast dynamics in atomic, molecular, and solid-state science, the need for ever faster carrier-envelope phase (CEP) detection and control has arisen. Here we demonstrate a high-speed, continuous, every-single-shot measurement and fast feedback scheme based on a stereo above-threshold ionization time-of-flight spectrometer capable of detecting the CEP and pulse duration at a repetition rate of up to 400kHz. This scheme is applied to a 100kHz optical parametric chirped pulse amplification few-cycle laser system, demonstrating improved CEP stabilization and allowing for CEP tagging.